This section provides background information related to the present disclosure which is not necessarily prior art.
Hydrogen is an attractive fuel as it can provide low emissions and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device having an anode and a cathode separated by an electrolyte. The anode receives a fuel such as hydrogen gas and the cathode receives an oxidant such as oxygen or air. Hydrogen gas is dissociated in the anode to generate free protons and electrons, where the protons pass through the electrolyte to the cathode. The electrons from the anode do not pass through the electrolyte, but are instead directed through a load to perform work before being directed to the cathode. In the cathode, the protons, electrons, and oxygen react and generate water.
Proton exchange membrane (PEM) fuel cells are a type of fuel cell used to power vehicles. The PEM fuel cell generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode can include a catalytic mixture of finely divided catalytic particles, such as platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture can be deposited on opposing sides of the membrane. Combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane can be referred to as a membrane electrode assembly (MEA).
Several fuel cells can be combined into one or more fuel cell stacks to generate the desired power. For certain applications, a fuel cell stack may include several hundred or more fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen may be consumed by the stack, and some of the air can be output as a cathode exhaust gas that can include water as a stack byproduct. The fuel cell stack also receives an anode reactant gas such as hydrogen that flows into the anode side of the stack.
A fuel cell stack can include a series of bipolar plates positioned between several MEAs within the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates to allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates to allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates can also include coolant flow channels, through which a cooling fluid flows to control the temperature of the fuel cell.
Stack order switching or flow shifting can be used in a fuel cell system that employs split stacks. Particularly, suitable valves and plumbing in the system can be provided so that the anode exhaust gas exiting a first sub-stack is sent to the anode inlet of a second sub-stack, and the anode exhaust gas exiting the second sub-stack is sent to the anode inlet of the first sub-stack in a cyclical manner.
Distribution of hydrogen within the anode flow channels of the fuel cell stack can be kept substantially constant during fuel cell stack operation. To this end, more hydrogen is directed into the fuel cell stack than is necessary for a certain output load of the stack so that the anode gas is evenly distributed. However, the anode exhaust gas can subsequently include a significant amount of hydrogen gas that can reduce system efficiency if the hydrogen is simply discarded. The anode exhaust gas can therefore be recirculated back to the anode input to reuse the hydrogen.
MEAs are permeable and therefore allow nitrogen and other gases present in air on the cathode side of the fuel cell stack to permeate therethrough and collect in the anode side of the fuel cell stack. This is referred to as crossover. Even though the anode side pressure may be slightly higher than the cathode side pressure, cathode side partial pressures can cause gases within air to permeate through the membrane. For example, nitrogen entering the anode side of the fuel cell stack consequently dilutes the hydrogen fuel gas, and if the nitrogen concentration increases above a certain percentage, such as 50%, operation of the fuel cell stack can be affected. A bleed valve can be provided in an anode recirculation loop or the anode exhaust of the fuel cell stack to purge nitrogen and other diluent gases from the anode side of the stack, where they can be directed to an exhaust stream, such as the cathode exhaust.
Gas that is periodically bled from the anode recirculation loop or anode exhaust can include a considerable amount of hydrogen. As such, bled recirculation gas can be directed to a combustor to burn most or all the hydrogen therein before the recirculation gas is exhausted to the environment. However, the combustor adds complexity, cost, and weight to the fuel cell system. In some cases, bled recirculation gas can also be directed to the cathode upstream of the fuel cell stack.
Water can also migrate from the cathode side and collect on the anode side of the fuel cell stack, requiring a means to remove water from the anode side. A water separator including a valve located at the bottom of a sump, in conjunction with a liquid water level sensor, can be used to detect and remove liquid water condensate from the anode side where it can be routed to an exhaust stream, such as the cathode exhaust.
Removal of diluent gas and removal of water from the anode side by routing each to an exhaust stream provides a path for the hydrogen fuel gas to enter the exhaust stream, which may present a hydrogen emission concern.